The present invention relates generally to the field of raster scanning CRT visual display systems in which circuitry is utilized to provide for at least two different vertical spacings between sequential horizontal scan lines in a raster scan visual display system.
In CRT raster scanning visual display systems, the information to be displayed typically is provided by a microprocessor. This information is normally supplied to a CRT controller circuit which generates the horizontal and vertical display sync pulses as well as a display timing signal that determines the horizontal and vertical video blanking times which correspond to the occurrence of the horizontal and vertical sync pulses. In addition, the typical visual display system also utilizes a refresh memory circuit wherein information concerning the visual display to be provided in a single display frame is received from the microprocessor and stored for later recall by the operation of the CRT controller circuit. Also, such systems utilize a character generator which receives control signals from the CRT controller circuit and display information signals from the refresh memory circuit and in response thereto provides scan excitation signals to a video control circuit that determines the video excitation to be provided on the CRT screen. This video excitation comprises the timed activation of a scanning CRT electron beam gun to produce a desired display character at a desired location on the CRT screen. The operation of the above-identified basic visual display system is well known to those of average skill in the video display system art, and therefore additional details explaining the operation of such systems are not believed to be necessary.
The present invention is more particularly related to adjustment circuits which alter the normal vertical spacing between seqential horizontal raster scan lines provided for the CRT visual display. Typically the video beam, or video beams in the case of a color CRT display, are swept horizontally across the CRT screen from left to right during an active portion of a display timing signal. During a non-active portion, the video gun beam follows a retrace path to a slightly lower initial left side position. In accordance with the occurrence of a subsequent sync pulse the beam again implements an active scan across the CRT screen but this time at a slightly altered vertical position. At the end of scanning the entire CRT screen to produce a visual display frame, a vertical sync pulse is received by the CRT resulting in resetting the beam to the top left hand corner of the screen and the entire process is repeated.
It is known that in some instances it is desired to provide a larger than normal vertical space between sequential active video horizontal lines of video information provided by the CRT video gun. This permits the CRT to display desired information in a desired visual format. Preferably this feature should be implemented without requiring the refresh memory to repetitively provide blank video signal information for each blank horizontal scan line to be implemented to create a larger than normal vertical spacing between video active horizontal scan lines. One such system which provides an adjustable vertical spacing between horizontal scan lines is illustrated in U.S. Pat. No. 4,019,909 which is assigned to the same assignee as the present invention. In this prior system typically a vertical shift for a larger than normal vertical space between sequential horizontal scan lines is implemented by a separate vertical deflection control signal during a single retrace time of the video beam in order to minimize disruption of the visual display. However, many times a substantial increase in the normal vertical spacing between horizontal lines is desired such that this increased vertical spacing cannot be implemented during the short duration retrace time. This can be due to the vertical deflection circuits of the CRT being unable to rapidly respond to a vertical step control signal during the short retrace time.
The present invention provides for selectively implementing a larger than normal vertical step while utilizing a standard CRT controller circuit and preventing any disruption of the produced visual display. The present invention also provides a selective vertical spacing adjustment circuit which, while it can be operated independently to provide a larger than normal vertical spacing, can also be utilized in conjunction with prior vertical step adjustment circuits to insure the proper operation of these prior circuits even when the normal response time of the vertical deflection control apparatus would prevent the use of such prior circuits to provide a completed large desired vertical step just during the short retrace time between horizontal scan lines.